Field of the Invention
The present invention relates to an analog multiplier, notably for a precision electronic wattmeter operating according to the time-division multiplication method and comprising a modulator, a first switch and a low-pass filter.
The modulator is adapted to yield a cyclic ratio .theta..sub.x of a value proportional to a first analog input signal x representative of a multiplier when this signal x is fed to the modulator input.
Said first switch is responsive to the cyclic ratio .theta..sub.x and receives on a first input a second analog input signal y representative of a multiplicand, and on another input the same signal but having its sign inverted, i.e. -y.
The filter is connected to the output of said first switch. Therefore, it receives during a time period t.sub.1 the signal +y and during a time period t.sub.2 the signal -y, said times t.sub.1 and t.sub.2 being such that ##EQU1## WITH PREFERABLY T.sub.1 + T.sub.2 = CONSTANT.
Thus, the filter output delivers a continuous signal substantially equal to the average value of the instantaneous scalar product of the two signals x and y.
Multipliers of this character are incorporated notably in electronic wattmeters. For the sake of convenience, the following description will refer to a wattmeter, but it will readily occur to those conversant with the art that the analog multiplier according to this invention can be used with any other apparatus and appliance in which an analog multiplication of two input signals is required.
As a rule, these two input signals are alternating ones. They comprise as a rule a fundamental component of same frequency. Furthermore, these input signals can be phase-shifted to each other and have extremely distorted waveforms, possibly with a continuous component.
When signals at 400 Hz or even 2,000 Hz have to be processed with a relatively high degree of precision, of the order of, say, 10.sup.-5, the chopping performed by the modulator should have a high frequency of the order of 200 kHz.
However, it is obvious that at such high frequencies errors are caused by the switching action. Thus:
On the one hand, switching dispersions and delays, both when switching on and when switching off, introduce errors that are independent of the signal level, and PA1 on the other hand, the rise and fall times of the signals constitute other sources of errors depending on the signal level, thus impairing linearity and reducing the range of practical applications.
Finally, in the case of continuous input signals, errors are caused by the shift voltage of amplifiers constituting compulsory components of such electronic wattmeters. Now these shift errors could be minimized by using chopping amplifiers or choppers, but the cost of chopping amplifiers is considerably higher than that of integrated-circuit operational amplifiers.
While a wattmeter can operate continuously without any error increment, it is possible to calibrate the instrument continuously, which constitutes an important feature, since accurate d.c. sources are more easily obtained and it is no more necessary to take into consideration possible phase shifts or waveform distortions.